This invention relates to photopolymers, and, more particularly, to a photopolymer composition and its polymerization procedure.
Photopolymers are polymers that are formed from constituents such as monomers as a result of the energy introduced by shining light into the constituents. A number of monomers have been used in photopolymerizing systems, with the acrylates and methacrylates the most popular. The mixture of constituents also includes an initiator, which is a reactant or combination of reactants that, when exposed to light, is activated and causes the polymerization of the other constituents.
Photopolymers are widely used to selectively record patterns and images. Some applications include the patterning of semiconductor substrates in the microelectronics industry and the preparation of holographic images. For example, in the preparation of a hologram, a sheet of the photopolymerizing mixture is exposed to properly configured laser light, causing selectively variable polymerization of the mixture. After the polymerized sheet is developed, the three-dimensional image is projected using laser light. In most circumstances, it is preferred to use the same wavelength of light for projection as was used for exposure.
Existing photopolymerizing systems have two major shortcomings when considered for use in many commercial applications. First, the final polymer is sensitive to moisture. The polymer tends to absorb water, either when immersed or from the water vapor in the air. In the case of a hologram, the absorption of water can distort the image that is recorded in the polymerizod material. It is therefore necessary to seal the polymer material containing the hologram hermetically shortly after the hologram is developed and the developer removed. Sometimes, however, hermetically sealing the hologram is not practical, either because of the nature of the piece of the polymer material containing the hologram, or because there is too great a likelihood that the hermetic seal will be broken during the intended life of the hologram. The cost of providing the required hermetic seal may be prohibitive. Finally, even with careful attention to developing and sealing the hologram, residual water from the developing procedure may eventually distort the image.
The second shortcoming with existing photopolymer systems is that, in many cases, they are sensitive to light of wavelengths that are not practical for use in some applications. Various available photopolymerizing systems are sensitive to a range of wavelengths, but in most cases are not sensitive to red light, such as produced by a helium-neon laser operating at 6328 Angstroms. In some potential applications the use of red light to project the hologram would be desirable, particularly where the color red has a traditional association in the minds of most persons. Red light also has the advantage that it has a lesser effect on reduction of night vision than do other wavelengths of visible light. For example, if a holographic image were to be used in either a warning system, a night-vision system, or an instrument panel that might be used at night, red projecting light would be preferred. The photopolymer system used to record the image would therefore desirably be sensitive to red light, so that red light could be used both for exposure of the holographic image and for its projection.
There is a need for a photopolymerizing system suitable for imaging applications, which is both inherently insensitive to water degradation and also is sensitive to polymerization under the action of red light, but no such system is now available. The present invention fulfills this need, and further provides related advantages.